A typical liquefaction process is described in U.S. Pat. No. 6,272,882 in which the gaseous, methane-rich feed is supplied at elevated pressure to a first tube side of a main heat exchanger at its warm end. The gaseous, methane-rich feed is cooled, liquefied and sub-cooled against evaporating refrigerant to get a liquefied stream. The liquefied stream is removed from the main heat exchanger at its cold end and passed to storage as liquefied product. Evaporated refrigerant is removed from the shell side of the main heat exchanger at its warm end. The evaporated refrigerant is compressed in at least one refrigerant compressor to get high-pressure refrigerant. The high-pressure refrigerant is partly condensed and the partly condensed refrigerant is separated into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction. The heavy refrigerant fraction is sub-cooled in a second tube side of the main heat exchanger to get a sub-cooled heavy refrigerant stream. The heavy refrigerant stream is introduced at reduced pressure into the shell side of the main heat exchanger at an intermediate point with the heavy refrigerant stream being allowed to evaporate in the shell side of the main heat exchanger. At least part of the light refrigerant fraction is cooled, liquefied and sub-cooled in a third tube side of the main heat exchanger to get a sub-cooled light refrigerant stream. This light refrigerant stream is introduced at reduced pressure into the shell side of the main heat exchanger at its cold end, and the light refrigerant stream is allowed to evaporate in the shell side.
It is apparent from the description provided above that the tube side of the main heat exchanger is required to handle three streams, namely: i) a gaseous, methane-rich feed which enters the warm end of the first tube side as a gas at elevated pressure, condenses as it travels through the first tube side, and leaves the cold end of the first tube side as a sub-cooled liquefied stream; ii) a heavy refrigerant fraction which enters the warm end of the second tube side as a liquid, is sub-cooled as it travels through the second tube side, and leaves the cold end of the second tube side as a sub-cooled heavy refrigerant stream; and, iii) a least a part of the light refrigerant fraction which enters the warm end of the third tube side as a vapour, is cooled, liquefied and sub-cooled as it travels through the third tube side, and leaves the cold end of the third tube side as a sub-cooled light refrigerant stream.
At the same time, the shell side of the main heat exchanger is required to handle: a) a heavy refrigerant stream which enters the shell side at an intermediate location (at a location referred to in the art as the “top of the warm tube bundle”), and which is evaporated within the shell side before being removed as a gas from the shell side at its warm end; and, b) a light refrigerant stream which enters the shell side at reduced pressure at its cold end (at a location referred to in the art as the “top of the cold tube bundle”), and which is evaporated within the shell side before being removed as a gas from the shell side at its warm end.
Thus, in order to operate in the type of liquefaction process described in U.S. Pat. No. 6,272,882, the main heat exchanger must be capable of handling both single and two phase streams, all of which condense at different temperatures, with multiple tube-side and shell-side streams being accommodated in the one exchanger. The main heat exchanger must also be capable of handling streams having a broad range of temperatures and pressures. For this reason, the main heat exchanger used in liquefaction plants around the world is a “coil-wound” or “spiral-wound” heat exchanger.
In such coil-wound heat exchangers, the tubes for each of the individual streams are distributed evenly in multiple layers which are wound around a central pipe or mandrel to form a “bundle”. Each of the plurality of layers of tubes may comprise hundreds of evenly sized tubes with an even distribution of each of the first, second and third tube side fluids in each layer in proportion to their flow ratios. The efficiency of the main heat exchanger relies on heat transfer between the shell side and the tube side in each of these multiple layers being as balanced as possible—both radially across the bundle and axially along the length of the bundle.
As spiral-wound heat exchangers become larger to perform increased duties, it becomes increasingly difficult to distribute the shell side fluids evenly. This is partly due to the fact that on the shell side, the composition of the heavy and light refrigerant streams change continuously along the length of the main heat exchanger as the light components of the refrigerant boil off first. As a consequence, heat transfer between the shell side and each of the first, second and third tube sides may become uneven across the layers within the bundle. This uneven distribution of temperature in the shell side fluids leads to unevenness in the temperature in portions of each of the tube side fluids at the cold ends of the bundle from each layer of tubes in the bundle, and for the shell-side fluid exiting at the warm end.
When the system is in balance, the temperature difference between the tube sides and the shell side remains relatively constant but narrow along the majority of the length of the main heat exchanger. When the system is out of balance, the close temperature differential between the tube sides and the shell side can become “pinched” at locations where a very small or no temperature differential exists at all. Such pinching causes a drop in efficiency of the main heat exchanger. A consequential drop in efficiency is also experienced in the associated mixed refrigerant compression circuit which receives the fluid exiting the warm end of the shell side of the main heat exchanger. If the main heat exchanger is working correctly, the fluid exiting the warm end of the shell side is a gas. When the main heat exchanger is out of balance, the fluid exiting the warm end of the shell side may comprise a two phase mixture of gas and liquid. Any liquid present represents a significant loss of efficiency and must also be removed to avoid potential damage to the downstream refrigerant compression circuit.
The present invention provides a process and apparatus for improving the efficiency of a main heat exchanger by overcoming at least one of the problems identified above.